JP2011214137A - Method for controlling concentration of s-compound-containing solution for treating cold rolled steel sheet and method for producing cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling concentration of an S-compound-containing solution for treating a cold rolled steel sheet, and a method for producing the cold rolled steel sheet, in which it is possible to reduce the amount of an S compound used for production of a high strength cold rolled steel sheet and to reduce a production cost, while stably producing the high strength cold rolled steel sheet.SOLUTION: The method for controlling concentration of an S-compound-containing solution for treating a cold rolled steel sheet includes: measuring the concentration of an S-compound in the S-compound-containing solution using an analytical means having a minimum determination limit of 0.70 mg-S/L or less in terms of S, when annealing a cold rolled steel strip containing 0.8-3.0 mass% of Si, pickling the annealed strip, and then performing a treatment using the S-compound-containing solution; and controlling the concentration of the S-compound in the S-compound-containing solution to 3.5 mg-S/L or more in terms of S, based on the measured concentration of the S-compound. There is also disclosed a method for producing a cold rolled steel sheet using the method for controlling concentration.

Description

  The present invention relates to a method for controlling the concentration of a sulfur compound-containing solution within an appropriate range when a cold-rolled steel sheet is processed using the sulfur compound-containing solution. The present invention also relates to a method for producing a cold-rolled steel sheet having a tensile strength of 590 MPa or more using the concentration control method.

  In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of conservation of the global environment, while an improvement in automobile safety has also been demanded from the viewpoint of passenger protection in the event of a collision. For this reason, it is necessary to reduce the weight and increase the strength of the automobile body. Recently, the reduction in the thickness and the increase in the strength of the automobile parts has been promoted.

  Here, since many automobile parts are manufactured by press-molding steel sheets, excellent press formability and high strength are strongly demanded for steel sheets used for automobile parts. As a method for obtaining a steel sheet having high strength without greatly impairing press formability, a solid solution strengthening method by adding Si is known.

However, in the solid solution strengthening method, when a cold rolled steel sheet contains a large amount, particularly 0.8 mass% or more, of Si oxide such as SiO ₂ (silicate) or SiMnO ₃ (manganese silicate) during annealing. Will be formed on the surface of the steel sheet. These Si oxides inhibit the etching of the steel sheet surface and inhibit the formation of a healthy chemical conversion film in the zinc phosphate treatment (chemical conversion treatment) performed as a base treatment for electrodeposition coating of the steel plate. Therefore, such high-strength cold-rolled steel sheets with a high Si content can be applied to ordinary steel sheets when exposed to harsh environments such as a salt warm water immersion test or a combined cycle corrosion test that repeats wet-drying after electrodeposition coating. In comparison, the coating film is easily peeled off, and the corrosion resistance after coating tends to be lowered.

  Therefore, the zinc phosphate crystals generated at the initial stage of the chemical conversion treatment by removing Si oxide from the steel sheet surface by pickling and making a predetermined amount of sulfur compound (hereinafter referred to as “S compound”) present on the steel sheet surface. There has been proposed a method for improving the corrosion resistance after coating by increasing the number of nuclei to refine and refine the zinc phosphate crystal and improve the phosphoric acid treatment property (chemical conversion property) of the steel sheet. Specifically, a steel slab containing 0.8 to 3.0% by mass of Si is hot-rolled, cold-rolled, continuously annealed, and then pickled on the surface of the steel plate. There has been proposed a method for producing a high-strength cold-rolled steel sheet having excellent post-coating corrosion resistance by contacting with an aqueous solution containing bismuth (see, for example, Patent Documents 1 to 3).

JP 2007-126747 A JP 2007-217743 A JP 2007-224325 A

  Here, in the conventional method for producing a high-strength cold-rolled steel sheet having excellent post-coating corrosion resistance, the concentration of the aqueous solution containing the S compound has not been particularly controlled. However, according to the studies by the present inventors, in the conventional manufacturing method described above, the S compound is adsorbed on the steel sheet surface by bringing the steel sheet immediately after pickling into contact with the S compound-containing solution. When a high strength cold-rolled steel sheet is continuously produced, the S compound concentration in the S compound-containing solution decreases with the passage of time, and a high-strength cold-rolled steel sheet having excellent corrosion resistance after coating can be stably produced. It became clear that it might disappear.

  Therefore, the inventors of the present invention provide a high-strength cold-rolled steel sheet that is excellent in post-coating corrosion resistance by appropriately managing the concentration of the S-compound-containing solution in the method for producing a high-strength cold-rolled steel sheet using treatment with an S compound-containing solution. In order to reduce the manufacturing cost by reducing the amount of S compound used for the production of a high-strength cold-rolled steel sheet while stably producing the steel sheet, intensive studies were conducted.

Specifically, the present inventors first examined the lower limit of the concentration of the S compound-containing solution necessary for improving the chemical conversion treatment property by attaching a sufficient amount of S compound to the steel sheet surface. The following experiments were performed using high-strength steel (Si content: 1.2 mass%) with a strength of 590 MPa and high-strength steel (Si content: 1.5 mass%) with a strength of 780 MPa.
First, after annealing, each high-strength steel is pickled with a 20% by mass nitric acid-5% by mass hydrochloric acid solution, and then immersed in an S-compound-containing solution having each concentration shown in Table 1 below. Got. Next, the obtained steel sheet was sequentially subjected to spray degreasing treatment, surface conditioning treatment, and chemical conversion treatment, and then an electrodeposition coating was applied to prepare a test piece. And after giving a crosscut wrinkle with a cutter with respect to the test piece, the test piece was immersed in the 60 mass C 5 mass% NaCl solution for 240 hours, Then, water washing and drying were performed. And finally, the tape peeling test was done with respect to the crosscut collar part of the test piece, and the maximum peeling width of the right and left of the crosscut collar part was measured.

  As a result, the maximum peel width of the test piece is as shown in Table 1, and a sufficient amount of S compound is attached to the steel sheet surface to improve the chemical conversion treatment property, and continuously cold-rolled steel sheet having excellent corrosion resistance after coating. In order to obtain stably, it became clear for the first time in this examination that it is necessary to use the S compound containing solution whose S compound concentration is 3.5 mg-S / L or more in terms of sulfur (S conversion).

Therefore, next, the present inventors examined the means necessary for controlling the concentration of the S compound in the S compound-containing solution to 3.5 mg-S / L or more when producing a high-strength cold-rolled steel sheet. Went. As a result, in order to control the concentration of the S compound in the S compound-containing solution to 3.5 mg-S / L or more, the concentration of the S compound is measured with an accuracy of about 3.5 ± 0.2 mg-S / L. And found it necessary to manage. Therefore, as a result of further investigation based on the inventors' previous experience regarding process control analysis, in order to measure the concentration of S compound with an accuracy of about 3.5 ± 0.2 mg-S / L, a blank is used. The standard deviation (σ) when repeatedly measuring is 0.07 mg-S / L or less (that is, the lower limit of quantification corresponding to 10 times the standard deviation (σ) is 0.70 mg-S / L or less). It has been found that it is necessary to use analytical means.
And the present inventors completed this invention based on the said knowledge.

  That is, this invention aims at solving the said subject advantageously, The concentration control method of the S compound containing solution for cold-rolled steel plate processing of this invention is Si 0.8-3.0 mass. After annealing and pickling the cold-rolled steel strip, the S compound concentration in the S compound-containing solution is 0.70 mg when the lower limit of quantification is converted to S. -Measured using an analysis means of S / L or less, and based on the measured S compound concentration, the S compound concentration in the S compound-containing solution is controlled to be 3.5 mg-S / L or more in terms of S It is characterized by doing. In the present invention, the “lower limit of quantification” refers to a value obtained by multiplying the standard deviation (σ) of a measurement value obtained when a blank is measured 10 times repeatedly, and the “lower limit of quantification is 0.70 mg in terms of S”. “S / L or less” means that when a plurality of S compounds are added to the S compound-containing solution, the total value of the lower limit of quantification (S converted value) of each S compound in the analysis means used is 0. It means that it is 70 mg-S / L or less.

  Here, the method for controlling the concentration of the S compound-containing solution for cold-rolled steel sheet processing according to the present invention is such that the analysis means is an X-ray fluorescence analysis method, an ICP emission spectroscopy method, a liquid chromatograph method, an absorptiometry method or a combustion infrared ray. An analyzer using a sulfur analysis method is preferred. This is because an analytical apparatus using these analytical methods has a lower limit of quantification of 0.70 mg-S / L or less, and is particularly suitable for the concentration control method of the present invention.

  Moreover, the manufacturing method of the cold rolled steel sheet of this invention is the process of performing the process using an S compound containing solution, after annealing and pickling the cold rolled steel strip containing 0.8-3.0 mass% of Si. A method for producing a cold-rolled steel sheet, comprising controlling the concentration of S compound in the S compound-containing solution by using any one of the above-described concentration control methods.

  According to the method for controlling the concentration of the S compound-containing solution for cold-rolled steel sheet processing according to the present invention, the S compound concentration in the S compound-containing solution is controlled to 3.5 mg-S / L or more in terms of S, thereby improving the corrosion resistance after coating. The S compound concentration necessary for stably producing an excellent cold-rolled steel sheet can be ensured without adding an excessive amount of the S compound. Moreover, according to the manufacturing method of the cold-rolled steel plate of this invention, the cold-rolled steel plate excellent in corrosion resistance after coating can be manufactured stably at low cost.

It is explanatory drawing which shows typically the principal part of the typical cold-rolled steel plate manufacturing apparatus applicable to the manufacturing method of the cold-rolled steel plate of this invention. It is a graph which shows the time-dependent change of the S compound density | concentration in the S compound containing solution at the time of manufacturing a cold-rolled steel plate continuously using the apparatus shown in FIG. It is a graph which shows the time-dependent change of the S compound density | concentration in the S compound containing solution at the time of manufacturing a cold-rolled steel plate continuously using the apparatus of a comparative example. It is a graph which shows the time-dependent change of the S compound density | concentration in the S compound containing solution at the time of manufacturing a cold-rolled steel plate continuously over a long time using the apparatus shown in FIG.

  Embodiments of the present invention will be described below with reference to the drawings. Here, the manufacturing method of the cold-rolled steel sheet according to the present invention hot-rolled a steel slab containing 0.8 to 3.0% by mass of Si as described in Patent Documents 1 to 3, for example. Production of a cold-rolled steel sheet in which an S compound is present on the surface of the steel sheet by bringing the S compound-containing solution into contact with the cold-rolled steel strip obtained by cold rolling later, immediately after continuous annealing and pickling. In the method, the concentration of the S compound-containing solution brought into contact with the steel sheet is controlled by a predetermined concentration control method. And in an example of the manufacturing method of the cold-rolled steel plate of this invention, using the cold-rolled steel plate manufacturing apparatus which shows only the principal part in FIG. 1, for example, as follows, the high-strength cold-rolled steel plate with a tensile strength of 590 MPa or more Manufacturing. In the following example manufacturing method, hot rolling conditions, cold rolling conditions, annealing conditions, pickling conditions, and the like can be appropriately changed.

  First, a steel slab containing 0.8 to 3.0 mass% of Si is heated to, for example, a temperature of 1200 ° C. or less, hot-rolled, and wound at a temperature of, for example, 400 to 650 ° C., and then optionally an acid. Washing is performed to obtain a hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet is cold-rolled at a rolling reduction of, for example, 30 to 60% to form a cold-rolled steel strip, and then annealed at a temperature of 850 ° C. or lower. And the high-strength cold-rolled steel plate is continuously manufactured by sequentially passing the annealed steel plate (steel strip) through the pickling tank 1, the S compound treatment tank 2, and the water washing tank 3 as shown in FIG. To do. In the present invention, since Si in the steel slab affects the chemical conversion property of the steel sheet, only the Si content is limited, and other components are not particularly limited. , C: 0.05 to 0.25 mass%, Mn: 0.5 to 3.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, Al: 0.06 mass% The following may be contained. Hereinafter, the reasons for limitation of the above components will be described.

  C: C is an element effective for producing retained austenite, bainite, martensite and the like necessary for strengthening the steel structure. Therefore, 0.05 mass% or more is contained, but if the C content exceeds 0.25 mass%, weldability is deteriorated. Therefore, the C content is 0.05 to 0.25% by mass, preferably 0.10 to 0.20% by mass.

  Mn: Mn strengthens steel by solid solution strengthening, improves the hardenability of the steel, and has an action of promoting the formation of retained austenite, bainite, and martensite. Such an effect is recognized when the amount of Mn is 0.5% by mass or more, but when it exceeds 3.0% by mass, it is saturated and the cost is increased. Therefore, the amount of Mn is 0.5 to 3.0% by mass, preferably 1.0 to 2.0% by mass.

  P: P is a solid solution strengthening element, and is usually an element effective for obtaining a high-strength steel sheet. Therefore, it is preferably contained in an amount of 0.005% by mass or more. Reduce weldability. Therefore, the P content is 0.05% by mass or less, preferably 0.02% by mass or less.

  S: S precipitates as MnS in the steel and reduces the stretch flangeability of the steel sheet. Therefore, the S content is 0.01% by mass or less, preferably 0.005% by mass or less, more preferably 0.003% by mass or less.

  Al: Al is an element added as a deoxidizer in the steelmaking stage, and is an element effective for separating nonmetallic inclusions that reduce stretch flangeability as slag, so 0.01% by mass or more Although it is preferable to contain, if it exceeds 0.06 mass%, a raise of cost will be caused. For this reason, Al amount shall be 0.06 mass% or less. Preferably it is 0.02 to 0.06 mass%.

As mentioned above, although the basic component was demonstrated, the component described below can be suitably contained in this invention as needed.
Ti, Nb, V: Ti, Nb, V forms carbides and nitrides, suppresses the growth of ferrite in the heating stage during annealing, refines the structure, and remarkably improves formability, especially stretch flangeability Let Therefore, it is effective to contain at least one of these elements. At this time, in order to obtain the above effect, each element needs to be contained in an amount of 0.005% by mass or more. However, if each element exceeds 0.3% by mass, the yield strength YS increases due to precipitation strengthening, the formability decreases, and the retained austenite for expressing the TRIP effect decreases. Therefore, the amount of these elements is 0.005 to 0.3 mass%, preferably 0.01 to 0.2 mass%, respectively.

  Mo: Mo is an element having an effect of improving the hardenability of steel and promoting the generation of bainite and martensite. Such an action is recognized when the Mo amount is 0.005% by mass or more, but when it exceeds 0.3% by mass, the effect is saturated and the cost is increased. For this reason, the amount of Mo is 0.005-0.3 mass%, preferably 0.01-0.2 mass%.

  Ca, REM: Ca and REM have the effect of controlling the form of sulfide inclusions and improving the stretch flangeability of the steel sheet. Such an effect can be obtained by including at least one of these elements. At this time, each element needs to be contained by 0.001% by mass or more. However, the effect is saturated when it exceeds 0.1 mass%. Therefore, the amount of these elements is 0.001 to 0.1% by mass, preferably 0.001 to 0.05% by mass, respectively.

  The pickling tank 1 removes Si oxide formed on the surface of the steel sheet during annealing by, for example, pickling the steel sheet with a mixture of nitric acid and hydrochloric acid, hydrochloric acid or sulfuric acid, etc. This is to reduce the amount. And in the pickling tank 1, it does not specifically limit, For example, a cold-rolled steel plate is pickled by being immersed in a 1 mass% hydrochloric acid -25 mass% nitric acid solution at 30-70 degreeC for 5 to 20 seconds.

The S compound treatment tank 2 adsorbs, for example, 0.1 to 100 mg-S / m ² of the S compound in terms of S to the surface of the steel sheet by bringing the pickled steel sheet into contact with the S compound-containing solution. The steel plate pickled in the pickling tank 1 is continuously treated with the S compound-containing solution in the S compound treating tank 2 while it is in a wet state, that is, before the surface is dried. The Examples of S compound-containing solutions include thiourea, thioglycolic acid, dimethyl sulfide, and salts (sodium thioglycolate, ammonium thioglycolate, etc.) and derivatives (dithiodiglycolic acid, dimethyl disulfide, dithiocarb An S compound aqueous solution having a known composition in which one or a mixture of two or more selected from the group consisting of hydrates and the like is dissolved in pure water can be used. Moreover, the process of the steel plate using S compound containing solution can be performed by immersing a steel plate in S compound containing solution for 30 second, for example, without being specifically limited.

  Here, the S compound treatment tank 2 has a sampling means for sampling the S compound containing solution in the S compound treatment tank 2 and an analysis for measuring the S compound concentration in the S compound containing solution sampled by the sampling means. Means. The S compound treatment tank 2 has S compound concentration control means for controlling the concentration of the S compound containing solution in the S compound treatment tank 2 based on the S compound concentration in the S compound containing solution measured by the analyzing means. The S compound supply means for supplying the S compound to the S compound treatment tank 2, the pure water supply means for supplying pure water to the S compound treatment tank 2, the S compound supply means and the pure water supply And operation control means for controlling the operation of the means. Specifically, the S compound treatment tank 2 is provided with a filter 21 and a sampling pump 22 as sampling means, and an analysis device 24 as analysis means, and a high concentration S compound as S compound supply means. A solution tank 25 and a high-concentration S compound solution pump 26, and a pure water tank 27 and a pure water pump 28 as pure water supply means are provided. The operations of the high concentration S compound solution pump 26 and the pure water pump 28 are based on the S compound concentration in the S compound containing solution measured by the analysis device 24, and the analysis device 24, the high concentration S compound solution pump 26 and the pure water pump 28. It is controlled by a control unit 29 as operation control means electrically connected to the water pump 28.

  The filter 21 is for removing suspended matters and dust contained in the sampled S compound-containing solution to improve the measurement accuracy of the S compound concentration in the analyzer 24. A 0.45 μm membrane filter can be used. And the S compound containing solution in the S compound processing tank 2 sampled with the sampling pump 22 via the filter 21 is sent to the analyzer 24, and S compound concentration is measured.

  The analyzer 24 is an analyzer having a lower limit of quantification of 0.70 mg-S / L or less in terms of S, for example, X-ray fluorescence analysis, ICP emission spectroscopy, liquid chromatography, absorptiometry or combustion infrared An analyzer using an absorption sulfur analysis method can be used. Then, in the analyzer 24, the S compound concentration in the sampled S compound-containing solution is measured continuously or intermittently as follows, for example. In addition, since the kind of S compound mix | blended with the S compound containing solution is known, in the analyzer 24, the concentration of these known S compounds is indirectly analyzed by quantitatively analyzing the S concentration in the S compound containing solution. Can be measured automatically.

  First, in the case of using an analyzer utilizing fluorescent X-ray analysis, the acid concentration of the sampled S compound-containing solution should be adjusted as necessary, and then analyzed using a filter paper drip method or a liquid sample holder. In the case of a wavelength dispersion type device, the intensity of the SKα ray having a wavelength of 0.5373 nm is obtained, and in the case of an energy dispersion type device, the energy intensity of 2.307 keV (Kα1 line) or 2.306 keV (Kα2 line) is obtained. Thus, the S compound concentration can be quantified.

  In the case of using an analyzer utilizing ICP emission spectroscopy, after adjusting the acid concentration of the sampled S compound-containing solution as necessary, the S compound-containing solution is introduced into the device, and a wavelength of 182.036 nm or By determining the intensity of emission at 180.669 nm, the concentration of S compound can be quantified.

  When using an analyzer utilizing the liquid chromatographic method, for example, a peak measurement method is used by passing the sampled S compound-containing solution as a mobile phase through a column packed with an appropriate packing material as a stationary phase. Thus, the S compound concentration can be determined. Note that a pump, a pretreatment column, a suppressor, or the like may be used as necessary.

  When using an analyzer utilizing absorptiometry, after adjusting the pH of the sampled S compound-containing solution as necessary, an appropriate color reagent such as molybdate ion and the S compound-containing solution are mixed, By measuring the absorbance at the absorption maximum point of the colored complex, the S compound concentration can be quantified.

When using an analyzer utilizing the combustion infrared absorption sulfur analysis method, a predetermined volume of the sampled S compound-containing solution is dropped on the combustion boat and dried, and then the S compound remaining on the combustion boat is removed together with the combustion boat. subjected to a heating furnace, the combustion of S compounds, can be obtained S compound amount concentration from the infrared absorption amount obtained in accordance with the amount of generated SO _2.

  The control unit 29 controls the S compound-containing solution measured by the analyzer 24 so that the S compound concentration of the S compound-containing solution in the S compound treatment tank 2 is 3.5 mg-S / L or more in terms of S. This is for controlling the operations of the high-concentration S compound solution pump 26 and the pure water pump 28 based on the compound concentration. As the control unit 29, a computer or the like can be used. In addition, if the S compound concentration of the S compound-containing solution in the S compound treatment tank 2 is in a range that is 3.5 mg-S / L or more in terms of S, an arbitrary target concentration value is provided, and the S compound treatment tank Control may be performed so that the S compound concentration of the S compound-containing solution in 2 falls within a range of, for example, 0.8 to 1.2 times the target concentration value (target concentration value ± 20%). Specifically, the control unit 29 is configured such that when the S compound concentration approaches 3.5 mg-S / L, or when the concentration is less than 0.85 times the target concentration value (the target concentration value is less than −15%) When the concentration is lower than the target concentration value, the high concentration S compound solution pump 26 is activated to supply the S compound to the S compound treatment tank 2, and the supply of the S compound causes the S compound concentration to reach the target concentration value. When it exceeds 1.15 times (target concentration value + 15%) or greatly exceeds the target concentration value, the pure water pump 28 is activated to supply pure water to the S compound treatment tank 2. You may do it.

  And in the manufacturing method of the cold-rolled steel plate of this example, the steel plate contacted with the S compound containing solution in the S compound treatment tank 2 is washed with pure water in the water washing tank 3 and contains an excess S compound attached to the steel plate. Solutions and the like are removed.

  According to such a method of manufacturing a cold-rolled steel sheet, since the Si oxide formed on the surface of the steel sheet during annealing is removed by pickling, it is possible to suppress deterioration of the chemical conversion processability of the manufactured cold-rolled steel sheet. can do. Further, in this manufacturing method, after pickling, an S compound-containing solution is brought into contact with the steel sheet to adsorb the S compound on the steel sheet surface, and the cathode sites such as cementite and sulfide are removed from the steel sheet surface by the pickling. However, since the adsorbed S compound functions as a cathode site, the produced cold-rolled steel sheet can generate zinc phosphate crystal nuclei finely and densely during the chemical conversion treatment, and good chemical conversion treatment properties can be obtained. Therefore, the cold-rolled steel sheet manufactured according to the manufacturing method according to the present invention is excellent in post-coating corrosion resistance.

  Here, in the said manufacturing method, the S compound is made to adsorb | suck to the surface of a steel plate by immersing a steel plate in the S compound containing solution. Moreover, since the wet steel plate is immersed in the S compound-containing solution after pickling, the S compound-containing solution is diluted. Therefore, particularly when a cold-rolled steel sheet is produced continuously, the S compound concentration of the S compound-containing solution in the S compound treatment tank 2 decreases with the passage of time. In the production method, the concentration of the S compound in the S compound-containing solution is controlled according to the concentration control method of the present invention so that the concentration is 3.5 mg-S / L or more. Can be adsorbed. In addition, the S compound concentration in the S compound-containing solution during steel plate production can be stabilized, and it is not necessary to use a large amount of the S compound in order to ensure the S compound concentration necessary for the S compound treatment. A cold-rolled steel sheet having excellent post-corrosion resistance can be stably produced at a low cost.

  In addition, from the viewpoint of producing a cold-rolled steel sheet having excellent corrosion resistance after coating, it is necessary to maintain the S compound concentration in the S compound-containing solution at 3.5 mg-S / L or more. Even in the case of S compound-containing solutions in the region, the above-described production method controls the S compound concentration reliably and accurately by measuring the S compound concentration using an analysis means having a lower limit of quantification of 0.70 mg-S / L or less. can do.

  As mentioned above, although embodiment of this invention was described with reference to drawings, the concentration control method of this invention and the manufacturing method of a cold-rolled steel plate are not limited to the example mentioned above, A change can be added suitably.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to the following Example at all.

0.11% by mass of C, 1.25% by mass of Si, 1.55% by mass of Mn, 0.018% by mass of P, 0.001% by mass of S, and 0.032% by mass of Al, The remainder is a steel slab composed of Fe and inevitable impurities, using a cold-rolled steel sheet manufacturing apparatus whose main part is shown in FIG. 1, and then hot-rolled and cold-rolled, and then annealed, pickled, under the conditions shown in Table 2. A cold-rolled steel sheet was produced by treating with the S compound. Here, the line speed in the S compound treatment was set to 300 m / min.
Here, a membrane filter having a pore diameter of 0.45 μm is used as the filter, and a filter paper drip analysis apparatus using a fluorescent X-ray analysis method (lower limit of quantification: 0.5 mg-S / L) is used as the analysis apparatus. It was. Moreover, thioglycolic acid is used as the S compound, the target concentration value of the S compound in the S compound treatment tank is 4.5 mg-S / L, and the control range is ± 20% (3.6 to 5.4 mg-S / L). When the measured value of the S compound concentration in the analyzer is less than 4.0 mg-S / L, a high concentration S compound solution is supplied, and the measured value of the S compound concentration is 5.0 mg-S / L. When it exceeded L, the concentration of the S compound-containing solution brought into contact with the steel sheet was controlled by supplying pure water.
FIG. 2 shows changes with time in the S compound concentration of the S compound-containing solution in the S compound treatment tank.
Moreover, about the obtained cold-rolled steel sheet, 600 seconds after manufacture start (sample 1-a), 1200 seconds after manufacture start (sample 1-b), 1800 seconds after manufacture start (sample 1-c), manufacture Samples were cut out from the position after 2400 seconds (sample 1-d) from the start, and the corrosion resistance after coating of each sample was evaluated by the following method. The results are shown in Table 3.

A cold-rolled steel sheet was manufactured in the same manner as described above except that a cold-rolled steel sheet manufacturing apparatus having no sampling means, analysis means, and S compound concentration control means was used and the concentration of S compound was not controlled.
FIG. 3 shows changes with time in the S compound concentration of the S compound-containing solution in the S compound treatment tank.
Moreover, about the obtained cold-rolled steel sheet, 600 seconds after manufacture start (sample 2-a), 1200 seconds after manufacture start (sample 2-b), 1800 seconds after manufacture start (sample 2-c), manufacture Samples were cut out from the position after 2400 seconds (sample 2-d) from the start, and the corrosion resistance after coating of each sample was evaluated by the following method. The results are shown in Table 3.

<Corrosion resistance after painting>
First, spray degreasing treatment (degreasing agent concentration: 16 g / L, treatment temperature: 42 to 44 ° C., treatment time: 120 seconds), surface conditioning treatment (total alkalinity of the surface conditioning agent: 1.5 to 2. 5 points, treatment temperature: 20-25 ° C., treatment time: 30 seconds, chemical conversion treatment (total acidity of chemical conversion treatment agent: 21-24 points, free acidity: 0.7-0.9 points, accelerator concentration: 2 8 to 3.5 points, treatment temperature: 44 ° C., treatment time: 120 seconds), and then an electrodeposition coating was applied to prepare a test piece. In addition, the adhesion amount of the chemical conversion treatment film was 2.0 to 2.5 g / m ^2, and the film thickness of the electrodeposition coating was 25 μm.
Next, after giving a crosscut wrinkle with a cutter with respect to the test piece, the test piece was immersed in a 5 mass% NaCl solution at 60 ° C. for 240 hours, and then washed and dried. And the tape peeling test was done with respect to the crosscut collar part, and the maximum peeling width of the right and left of the crosscut collar part was measured. The results are shown in Table 3.

  2 and 3, it can be seen that according to the concentration control method of the present invention, the concentration of the S compound in the S compound-containing solution can be controlled within an appropriate range. Moreover, it can be seen from Table 3 that according to the method for producing a cold-rolled steel sheet of the present invention, a cold-rolled steel sheet having excellent post-coating corrosion resistance can be stably obtained at low cost. Similar results were obtained when thiourea or dimethyl sulfide was used as the S compound.

0.12% by mass of C, 1.45% by mass of Si, 2.55% by mass of Mn, 0.019% by mass of P, 0.008% by mass of S, 0.040% by mass of Al, The balance was obtained by analyzing and controlling the concentration of the S compound treatment liquid when a steel slab composed of Fe and inevitable impurities was produced in the same process as in Example 1 over a long period of time exceeding 25000 seconds. Here, the line speed in the S compound treatment was set to 85 m / min.
An ICP emission spectroscopic analyzer (lower limit of quantification: 0.05 mg-S / L) was used as the analyzer. In addition, as the measurement solution, the S compound treatment liquid to be analyzed is collected from the S compound treatment tank, and suspended substances such as dust and sludge in the S compound treatment liquid are filtered with a PTFE membrane filter (pore diameter 0.45 μm). After removal, pure and 10-fold diluted were used. At that time, the measurement solution after 10-fold dilution was prepared such that nitric acid was 4% by mass and yttrium (Y) was 1 ppm as an internal standard substance. Note that a series of solution preparations can be automated by combining a pump, a multi-way valve, and the like before introducing the analyzer.
As the S compound, thioglycolic acid was used. The target concentration value of S compound in the S compound treatment tank is 6.5 mg-S / L, the control width is ± 25% (4.9 to 8.1 mg-S / L), and the analysis is performed every 2700 seconds. It was. At that time, when the measured value of the obtained S compound concentration was less than 5.5 mg-S / L, a high concentration S compound solution was supplied, and the measured value of the S compound concentration was 7.5 mg-S / L. When it became super, the density | concentration of the S compound containing solution made to contact with a steel plate was controlled so that pure might be supplied. In addition, when adjusting the S compound concentration by supplying a high concentration S compound solution or pure water, an analysis was performed immediately after that in order to confirm the adjusted S compound concentration.
FIG. 4 shows the change with time of the S compound concentration in the S compound-containing solution in the S compound treatment tank. From FIG. 4, it is understood that the concentration of the S compound treatment liquid can be managed in an appropriate range even in a long-time actual machine operation exceeding 25000 seconds.

  According to the method for controlling the concentration of the S compound-containing solution for cold-rolled steel sheet processing of the present invention, the S compound concentration is excessively added to the S compound concentration necessary for stably producing a cold-rolled steel sheet having excellent post-coating corrosion resistance. It can be secured without. Moreover, according to the manufacturing method of the cold-rolled steel sheet of the present invention, a high-strength cold-rolled steel sheet having a tensile strength of 590 MPa or more can be stably manufactured at a low cost.

DESCRIPTION OF SYMBOLS 1 Pickling tank 2 S compound processing tank 3 Water washing tank 21 Filter 22 Sampling pump 24 Analyzer 25 High concentration S compound solution tank 26 High concentration S compound solution pump 27 Pure water tank 28 Pure water pump 29 Control part

Claims (3)

  After annealing a cold-rolled steel strip containing 0.8 to 3.0 mass% of Si and pickling, when performing the treatment using the S compound-containing solution, the S compound concentration in the S compound-containing solution is set. The lower limit of quantification was measured using an analysis means having a S conversion of 0.70 mg-S / L or less, and based on the measured S compound concentration, the S compound concentration in the S compound-containing solution was 3.5 mg in S conversion. A method for controlling the concentration of an S compound-containing solution for cold-rolled steel sheet treatment, which is controlled to be at least S / L.   2. The analysis device according to claim 1, wherein the analysis means is an analyzer using a fluorescent X-ray analysis method, an ICP emission spectroscopic analysis method, a liquid chromatographic method, an absorptiometric method, or a combustion infrared absorption sulfur analysis method. The density | concentration control method of the S compound containing solution for cold-rolled steel plate description of description. A method for producing a cold-rolled steel sheet, comprising a step of performing a treatment using an S compound-containing solution after annealing and pickling a cold-rolled steel strip containing 0.8 to 3.0% by mass of Si. ,
A method for producing a cold-rolled steel sheet, wherein the S compound concentration in the S compound-containing solution is controlled using the concentration control method according to claim 1 or 2.

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